I have the exact same problem, but I also had it with iTunes 8 after upgrading to Snow Leopard. So it seems like iTunes 8 and 9 has this problem under 10.6.
I've been having this issue since I upgraded to 8. I get a few seonds of choppy music (if that) and then nothing. It seems to be a common problem, I was hoping Apple would fix it with 9.0. Iv'e upgraded everything and still have the issues; very frustrating.
I've been having this problem for a while under 10.5 and iTunes 8. I checked the Airport Express logs just then and noticed the drop-outs occur just after it updates from the time server. I have disabled automatic time updates and it seems to have fixed the problem, might work for you guys?
almost the same problem here. running mac os 10.6.1, itunes 9.1 and ae firmware 6.3, airtunes drops out frequently and sometimes constantly.
about the 'constantly' part, i think it could be a 'communication error' in the airport express itself, because it happens when a download reaches slightly over 300KB/s -> 2.34Mb/s. while itunes uses 133KB/s -> 1.04Mb/s. together not even close to max out the airport express 802.11g, with ca 20Mb/s thoughput [max 54Mb/s]. so why drop the airtunes connection?
for me it feels like certain download programs have a higher priority for using bandwidth on the airport express than itunes 9 and even safari.
but correct me if i'm wrong, i'm not a specialist.
i wasn't able to check if the 30 minute frequency drops still occur.
I'm running iTunes 9.0.1 through Airport Express and stereo receiver and get interruptions in my music. Is this a SL 10.6.1 bug with the latest iTunes? It didn't do this under previous iTunes editions!
Drop outs here too! Trying to deduce it out, so I'm testing out Airfoil to send iTunes to my Airport Express'. So far so good, I'll report back after a good long test. If Airfoil doesn't show itself to the same drop outs then it must be iTunes.
I had this problem with itunes 8. All was fine until a few tracks in and then I noticed the audio dropped.) At first I thought it was related to my airport express but it definitely happened when I wasn't using AE and it always happened inbetween songs. After advice on these forums, I downgraded to itunes 7 and the problem disappeared. Sounds like 9 has the same bug - so i guess I won't be upgrading!
My airport express also stopped playing music between songs. iTunes would keep playing but you couldn't hear anything on the speakers. If you paused the song and then un-paused it, it would play again. What finally worked was to turn off genius. Sad because I liked Genius. This happened after Snow Leopard "upgrade" and itunes 9.